6.8 The Indirect Radiative Forcing of Tropospheric Aerosols

6.8.1 Introduction

Aerosols serve as cloud condensation and ice nuclei, thereby modifying the
microphysics, the radiative properties, and the lifetime of clouds. The physics
and chemistry of the indirect effect of aerosols is discussed in detail in Chapter
5. Only aspects directly relevant to quantifying the indirect radiative
forcing by aerosols are presented here. The aerosol indirect effect is usually
split into two effects: the first indirect effect, whereby an increase in aerosols
causes an increase in droplet concentration and a decrease in droplet size for
fixed liquid water content (Twomey, 1974), and the second indirect effect, whereby
the reduction in cloud droplet size affects the precipitation efficiency, tending
to increase the liquid water content, the cloud lifetime (Albrecht, 1989), and
the cloud thickness (Pincus and Baker, 1994). Until recently, the first indirect
effect has received much more attention than the second. IPCC (1994) and the
SAR only considered the first indirect effect. Shine et al. (1996) retained
a range of radiative forcing from 0 to -1.5 Wm-2 with no best estimate,
although a value of -0.8 Wm-2 was used for the year 1990 in the IS92a
scenario (Kattenberg et al., 1996). Here we review and discuss the various estimates
for the globally averaged aerosol indirect forcing available in the literature.
Because of the inherent complexity of the aerosol indirect effect, GCM studies
dealing with its quantification necessarily include an important level of simplification.
While this represents a legitimate approach, it should be clear that the GCM
estimates of the aerosol indirect effect are very uncertain. Section
6.8.2 investigates the indirect radiative forcing due to sulphate aerosols,
on which most efforts have concentrated, while other aerosol types are treated
in Section 6.8.3. Section 6.8.4
is devoted to alternative approaches, while Section 6.8.6
describes the aerosol indirect effects on ice clouds.

6.8.2 Indirect Radiative Forcing by Sulphate Aerosols

6.8.2.1 Estimates of the first indirect effect

The studies reported in Table 6.6 use different GCMs and
methods for computing the droplet number concentration (i.e., empirical relationships
between the sulphate mass and the cloud droplet number concentration, empirical
relationships between the sulphate aerosol number concentration and the cloud
droplet number concentration, or parametrization of cloud nucleation processes).
The forcing estimates for the first indirect effect from sulphate aerosols range
from -0.3 to -1.8 Wm-2, which is close to the range of 0 to -1.5
Wm-2 given in the SAR when only a few estimates were available.

Table 6.6: The global mean annual average
aerosol indirect radiative forcing from different global studies. Letters
P (prescribed) and C (computed) refer to off-line and on-line sulphate aerosol
calculations, respectively. CCN and CDN stand for cloud condensation nuclei
and cloud droplet number, respectively. In studies indicated by an asterisk,
the estimate in flux change due to the indirect effect of aerosols was computed
as the difference in top of atmosphere fluxes between two distinct simulations
and therefore does not represent a forcing in the strict sense (see text).
When several simulations are performed in the same study, "base" indicates
the baseline calculation, while the range of estimates is given in parenthesis.

Includes a parametrization of cloud nucleation processes.
Uses a mixture of pre-existing aerosols.

Feichter et al. (1997)

Sulphate

-0.76

C

Uses Boucher and Lohmann (1995) A' relationship.

Jones and Slingo (1997)

Sulphate

-0.55 to -1.50

P

Uses 2 different versions of the Hadley Centre model.

Lohmann and Feichter (1997)*

Sulphate

-1

-1.4 to -4.8

C

Uses Boucher and Lohmann (1995) A' relationship.

Rotstayn (1999)*

Sulphate

base -1.2 (-1.1 to -1.7)

base -1.0 (-0.4 to -1.0)

base -2.1 (-1.6 to -3.2)

P

Includes a (small) long-wave radiative forcing.

Jones et al. (1999)*a

Sulphate

-0.91

base -0.66

-1.18

C

Includes a (small) long-wave radiative forcing.
The two effects add non-linearly.

Kiehl et al. (2000)

Sulphate

-0.40 to -1.78

C

Ghan et al. (2001a)*

Sulphate

~50% for base

~50% for base

base -1.7 (-1.6 to -3.2)

C

Includes a parametrization of cloud nucleation.
Predicted aerosol size distribution.

Lohmann et al. (2000)*

Sulphate

base -0.4 (0 to -0.4)

C

Includes a parametrization of cloud nucleation
processes.

Carb.

base -0.9 (-0.9 to -1.3)

C

Sulphate and Carb.

-40% for base

-60% for base

base -1.1 (-1.1 to -1.9)

C

Chuang et al. (2000b)

Sulphate

-0.30

C

Includes a parametrization of cloud nucleation
processes. Includes the effect of BC absorption in clouds.

Carb.

base -1.51 (-1.27 to -1.67)

C

Sulphate and Carb.

-1.85

C

aThis model predicts
too low sulphate concentrations on average.

There is a tendency for more and more studies to use interactive (on-line)
rather than prescribed (monthly or annual mean) sulphate concentrations. Feichter
et al. (1997) pointed out that the first indirect effect calculated from monthly
mean sulphate concentrations is 20% larger than calculated from interactive
sulphate concentrations. Jones et al. (1999) found that the total indirect effect
was overestimated by about 60% when they used seasonal or annual mean sulphate
concentrations.

The various GCM studies show some disagreement on the spatial distribution
of the forcing, an example of which is shown in Figure
6.7h. The Northern to Southern Hemisphere ratio varies from 1.4 to 4 depending
on the models. It is generally smaller than the Northern to Southern Hemisphere
ratio of anthropogenic sulphate aerosol concentrations because of the higher
susceptibility of the clouds in the Southern Hemisphere (Platnick and Twomey,
1994; Taylor and McHaffie, 1994). The ocean to land ratio depends very much
on the method used to relate the concentration of sulphate mass to the cloud
droplet number concentration and on the natural background aerosol concentrations.
It was generally found to be smaller than unity (Boucher and Lohmann, 1995;
Jones and Slingo, 1997; Kiehl et al., 2000). Larger ratios, such as 1.6 (Chuang
et al., 1997) and 5 (Jones and Slingo, 1997), are reported in some of the sensitivity
experiments. Using a detailed inventory of ship sulphur emissions and a simple
calculation of the aerosol indirect effect, Capaldo et al. (1999) suggested
that a significant fraction of the effect over the oceans (-0.11 Wm-2,
averaged globally) could be due to ship-emitted particulate matter (sulphate
plus organic material). So far this source of aerosols has not been included
in the GCM studies.

Kogan et al. (1996, 1997) used the Warren et al. (1988) cloud climatology over
the oceans rather than a GCM to predict the indirect effect by sulphate aerosols
on cloud albedo. The cloud albedo susceptibility was evaluated from a large
eddy simulation model applied to stratocumulus clouds. They found an indirect
short-wave forcing of -1.1 Wm-2 over the oceans with a small hemispheric
difference of 0.4 Wm-2 (i.e., a Northern to Southern Hemisphere ratio
of about 1.4). In their study, the forcing had a strong seasonal cycle, with
the Southern Hemisphere forcing prevailing in some seasons.